The invention relates to drop forming methods and apparatus for forming liquid chemicals into small particles.
It is conventional to form liquid chemicals into small particles. Such a practice is performed, for example in the production of pharmaceuticals, flavors, fragrances, detergents, antioxidants, plastics stabilizers, surfactants, nutrients, and edible fats among other chemicals.
One common way of producing such particles is through the use of a so-called drop former, as disclosed for example, in Froeschke U.S. Pat. No. 4,279,579 which comprises a stationary inner cylinder, called a stator, and a rotary outer drum. The stator forms an inner chamber which contains a flowable chemical and is maintained under pressure greater than atmospheric pressure, e.g., 35-100 psi on average, depending upon the viscosity and desired flow rate, among other parameters. The stator has a row of holes disposed at a six o""clock position. The outer drum, which is mounted for rotation on the stator, has rows of holes extending around the entire periphery thereof. The holes must be formed by a drilling process using a mechanical drill, which adds considerably to the manufacturing costs, and necessitates that the particles be of circular cross section, even though other shapes might be preferred.
During rotation of the outer drum, the rows of holes thereof sequentially pass through the six o""clock position whereby they come into vertical alignment with the row of holes of the stator. When that occurs, the pressure within the inner chamber forces the flowable chemical through the aligned holes, and becomes deposited in the form of drops upon a transfer surface, e.g., a belt, cylinder or web. The drops harden into particles on the transfer surface and are eventually removed therefrom.
When such a device is employed to make particles of relatively large size, e.g., 3 to 12 mm in diameter, the uniformity of diameter from one particle to the next is relatively uniform.
However, when making smaller particles, the diameter uniformity drops off dramatically. That drop-off creates problems in connection with the manufacture of certain kinds of particles. For instance, in the manufacture of pharmaceuticals, uniform particle size is necessary to ensure proper dosing.
Also, in the manufacture of powder detergents, additives are mixed with the detergent. Uniformity in the size of the additive particles is desirable to prevent solids-separation in packaging.
Moreover, in the manufacture of some plastics, stabilizers must be added. The stabilizers can be provided in the form of particles which must be melted in order to be properly combined with the plastics. The melting is often performed in an extruder. If the stabilizer particles are too large, the extruder barrel must be increased to ensure that the particles are properly melted to achieve a homogeneous mixing with the plastics. If homogeneity is not achieved, a larger percentage of additive must be provided to ensure the presence of enough additive throughout the plastic.
Therefore, it would be desirable to achieve, in a relatively inexpensive manner, a uniformity of diameter when making small particles, especially particles in the range of about 0.01 to 3.5 mm diameter, or about 0.01 to 10 mm2 in cross-sectional area. It would also be desirable to be able to produce particles in shapes other than round.
The present inventors believe they know why the uniformity of particle diameter drops off when making the smaller particles with a drop former, and have developed a drop forming method and apparatus for overcoming that problem.
The present invention relates to a drop forming apparatus for forming a flowable chemical into particles. The apparatus comprises a screen mounted for movement, the screen being of a thickness in the range of 0.05 to 0.4 mm and forming an internal chamber kept at substantially atmospheric pressure. The chamber is adapted to contain a flowable chemical. The screen has openings formed therethrough for discharging the flowable chemical from the chamber, each opening having a cross-sectional area in the range of 0.01 to 1.1 mm2. A drive member is operably connected to the screen for moving the screen. A pressure member is disposed within the chamber adjacent a lower portion of an inner surface of the screen for creating a hydrodynamic pressure as the screen rotates, for pressing the flowable chemical through the openings to form the flowable chemical into drops. A transfer surface is disposed beneath the screen onto which the drops are transferred and harden into particles. A collector is provided for collecting the particles from the transfer surface.
Preferably, the screen comprises a cylinder which is rotatable about a horizontal axis.
The invention also pertains to a method of forming a flowable chemical into particles. The method comprises the steps of providing a screen which forms an internal chamber, the screen being of a thickness in the range of 0.05 to 0.4 mm and having openings formed therethrough, each opening having a cross-sectional area in the range of 0.01 to 1.1 mm2. A flowable chemical is contained within the chamber under substantially atmospheric pressure. The screen is moved while the flowable chemical is hydrodynamically pressured by a pressure member and caused to flow through the openings to form the flowable chemical into drops. The drops are deposited onto a transfer surface which is disposed beneath the screen, whereupon the drops harden into particles. The particles are then removed from the transfer surface.